Evaporator coil staging and control for a multi-staged space conditioning system

ABSTRACT

A space conditioning system comprising an evaporator subunit (ES) and control subunit (CS). The ES includes at least three evaporator stages in a pathway of air flow through the ES. First and second stages are adjacent and have major surfaces substantially parallel to each other. A third stage is located in the pathway before the first stage. A major surface of the third stage covers the major surface of the first stage in a same pathway direction. The major surfaces are substantially perpendicular to the pathway. The CS is configured to operate the evaporator subunit in at least one of two partial load cooling modes. In a first mode, the CS causes refrigerant to circulate through the first and second but not through the third stage. In a second mode, the control subunit causes the refrigerant to circulate through the first and third stage but not through the second stage.

TECHNICAL FIELD

This application is directed, in general, to space conditioning systems and method for conditioning the temperature and humidity of an enclosed space and, in particular, to multi-staged systems and methods of using the same.

BACKGROUND

Multistage central air conditioning systems often operate under partial load conditions for extended periods, for example, during the spring or fall. Under such partial load conditions, however, it is often difficult to control the humidity of the enclosed space, thereby making the space uncomfortable. Additionally, it is desirable to optimize the extent of temperature reduction and humidity control while minimizing the system's energy expenditure.

SUMMARY

One embodiment of the present disclosure is a space conditioning system for conditioning air within an enclosed space. The system comprises an evaporator subunit, the evaporator subunit including at least three evaporator stages in a pathway of air flow through the evaporator subunit. First and second ones of the evaporator stages are adjacent to each other and have major surfaces that are substantially parallel to each other. A third one of the evaporator stages is located in the pathway before the first evaporator stage. A major surface of the third evaporator stage covers the major surface of the first evaporator stage in a same direction of the pathway. The major surfaces of each of the evaporator stages are substantially perpendicular to the airflow pathway. The system also comprises a control subunit configured to operate the evaporator subunit in at least one of two partial load cooling modes. In a first mode, the control subunit causes refrigerant to circulate through the first evaporator stage and through the second evaporator stage, but not through the third evaporator stage. In a second mode, the control subunit causes the refrigerant to circulate through the first evaporator stage and said third evaporator stage but not through the second evaporator stage.

Another embodiment of the present disclosure is a method of method of conditioning air within an enclosed space. The method comprises sensing a temperature or a humidity of an enclosed space, and, determining whether or not a space conditioning system coupled to the enclosed space can reduce the temperature or the humidity to a target value when using the system under partial load conditions. When using the system under partial load conditions can achieve the target conditions, operating the evaporator subunit of the system in one of two modes. In a first mode, a refrigerant is circulated through a first evaporator stage and a second evaporator stage, but not through a third evaporator stage of the evaporator subunit. In a second mode, said refrigerant is circulated through the first evaporator stage and the third evaporator stage but not through the second evaporator stage.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 presents an schematic exploded perspective view of an example embodiment of selected portions of a space conditioning system of the disclosure;

FIG. 2 presents a cross-sectional view of the evaporator subunit portion of the example system presented in FIG. 1, along view line 2-2;

FIG. 3 presents an alternative embodiment of the system, corresponding to the same portion of the system depicted in FIG. 2;

FIG. 4 presents another alternative embodiment of the system, corresponding to the same portion of the system depicted in FIG. 2; and

FIG. 5 presents a flow diagram of an example method of conditioning air within an enclosed space, such as implemented by any of the systems depicted in FIGS. 1-4.

DETAILED DESCRIPTION

Embodiments of the present disclosure provide a space conditioning system for conditioning air within an enclosed space and methods of efficiently operating the system. By controlling the flow of refrigerant through multiples stages of an evaporator subunit of the system under partial load conditions, cooling and humidity control of the enclosed space can be optimized.

One embodiment of the present disclosure is a FIG. 1 presents a schematic exploded perspective view of an example embodiment of selected portions of a space conditioning system 100 of the disclosure. FIG. 2 presents a cross-sectional view of the evaporator subunit portion of the example system presented in FIG. 1, along view line 2-2.

The system 100 comprises an evaporator subunit 105 than includes at least three evaporator stages 110, 112, 114 in a pathway 115 of air flow through the evaporator subunit. 105. As illustrated in FIG. 1, first and second ones of the evaporator stages (e.g., first evaporator stage 110 and second evaporator stage 112) are adjacent to each other and have major surfaces 117, 118 that are substantially parallel to each other. A third one of the evaporator stages (e.g., third evaporator stage 114 is located in the airflow pathway 115 before the first evaporator stage 110. A major surface 119 of the third evaporator stage 114 covers the major surface 117 of the first evaporator stage 110 in a same direction of the pathway 115. The major surfaces 117, 118, 119 of each of the evaporator stages 110, 112, 114, are substantially perpendicular to the airflow pathway 115.

The system 100 also comprises a control subunit 120 configured to operate the evaporator subunit 105 in at least one of two partial load cooling modes. In a first mode, the control subunit 120 causes a refrigerant to circulate through the first evaporator stage 110 and through the second evaporator stage 112 (e.g., the refrigerant delivered via first and second discharge lines 122, 124) but not through the third evaporator stage 114. In a second mode, the control subunit 120 causes the refrigerant to circulate through the first evaporator stage 110 (e.g., via the first discharge line 122) and the third evaporator stage 114 (e.g., delivered via a third discharge line 126) but not through the second evaporator stage 112. One skilled in the art would understand how electric devices of the control subunit 120 could be programmed to selectively open or close valves 128 connected to the discharge lines 122, 124, 126 to control the flow of refrigerant to the evaporator stages 110, 112, 114.

As illustrated in FIG. 1, in some embodiments, the evaporator stages 110, 112, 114 can be externally shaped as rectangular bodies with heat exchange fins 130 and one or more rows of a serpentine shaped hollow coil 132 (for clarity, only portions of which are depicted in FIG. 1) through the refrigerant is circulated. In other embodiments, the coil 132 can be a micro-channel coil. One skilled in the art would be familiar with other suitable configurations of the evaporator stages.

As noted above, the major surface 119 of the third evaporator stage 114 covers the major surface 117 of the first evaporator stage 110 in the same direction as the airflow pathway 115. That is, the major surfaces 117, 119 are sized and aligned such that substantially all of the air flowing to the first evaporator stage 110 has already passed through the third evaporator stage 114. For instance, configuring the outer perimeter of the major surface 117 of the first evaporator stage 110 to be the equal to or smaller than the outer perimeter of the major surface 119 of the third evaporator stage 114 helps ensure the desired coverage. For example, for rectangular-shaped evaporator stages, configuring the height 135 and width 137 of the third evaporator stage 114 to be respectively equal to, or greater than, the height 140 and width 142 of the first evaporator stage 110, helps to ensure coverage.

Although the control subunit 120 is configured to operate the evaporator subunit 105 in one of two partial load cooling modes, in other cases, the control subunit 120 configured to operate the evaporator subunit 120 in additional cooling modes. For instance, in some cases, such as on a hot summer day, the control subunit 120 is configured to operate the evaporator subunit 120 in a third mode (e.g., a full cooling mode in some cases), wherein the control subunit 120 causes the refrigerant to circulate through all of the evaporator stages (e.g., stages 110, 112, 114 in some embodiments).

Some embodiments are configured to improve the cooling or dehumidification efficiency for a given amount of power consumption when operating under particular partial load conditions. One skilled in the art would be familiar with the methods to quantify the efficiency of the system 100 under partial load conditions using metrics such as the Integrated Energy Efficiency Ratio (IEER) or the Integrated Partial Load Value (IPLV) as promulgated by standards associations such as the Air-Conditioning, Heating, and Refrigeration Institute.

In some embodiments, for example, a saturation temperature of the refrigerant in one or more suction lines 145, 147, 149 leaving the evaporator subunit 105 is higher when the subunit 105 is operating in the partial load first mode as compared to the second mode. The saturation temperature of the refrigerant in one or more suction lines is often referred to as the evaporator saturation temperature and is synonymous with the boiling point of the refrigerant in the suction line.

When operating in the first mode, their will be warm air contacting the first stage 110 directly because the third stage 114 is not used. But when operating in the second mode, because the third stage 114 is used and covers the first stage 110, the first stage 110 will receive air that has already been cooled by passing through the third stage 114. Therefore, the evaporator saturation temperature of the refrigerant in one or more suction lines leaving said evaporator subunit (e.g., the first suction line 146) will be higher when operating in the first mode than in the second mode.

Additionally, in some embodiments, when operating in the first mode, the evaporator saturation temperature of the refrigerant in the suction line 147 of the second evaporator stage 112 can be higher than the evaporator saturation temperature of the refrigerant in the suction line 149 of the third evaporator stage 114, when operating in the second mode. This can be the case when the second stage 112 has a greater capacity than the third stage 114, e.g., because the total effective surface area (e.g., the effective heat transfer surface) of the coil 132 in the second stage 112 is greater than the total effective surface area of the coil 132 in the third stage 114. For instance, as depicted in FIG. 1, in some embodiments, the second stage 112 coil 132 can have a greater number of rows than the third stage 114 coil 132. Of course, in other instances, the second and third stages 112, 114 have the same number of rows but the second stage's 112 coil 132 can still has a greater effective surface area, e.g., because it has larger dimensions (e.g., longer) than the third stage's 112 coil.

Moreover, because there is a one-to-one relation between the pressure and the saturation temperature, the higher temperature, the higher the pressure in one or more suction lines (e.g., one or more of lines 146, 147, 149). The higher saturation temperature is therefore conducive to a more efficient system 100 because, at the corresponding higher pressure associated with the first mode, the performance of a compressor subunit 150 (e.g., one or more compressors 152, 154, 156, 158) coupled to the evaporator subunit 105 is increased. Therefore, for a given amount of power consumed, the system 100 would provide more cooling capacity to the enclosed space when operating in the first mode.

In some embodiments, the enclosed space that the system 100 is configured to condition has a greater humidity removal capacity when the evaporator subunit 105 is operating in the second mode than when operating in the first mode. That is, the latent capacity (or dehumidification capacity) of the system 100 for a given amount of power consumption is greater when operating in the second mode as compared to the first mode. Consequently, in some cases, the enclosed space that the system 100 is configured to condition can have a lower humidity, or, reach a lower humidity target value faster, when the evaporator subunit 105 is operating in the second mode than when operating in the first mode.

The systems' 100 latent capacity when operating in the second mode is related to lowering (compared to the first mode) the refrigerant's pressure and evaporator saturation temperature in the one or more suction lines of the evaporator substage (e.g., one or more of lines 145, 147, 149). When there is a high suction line pressure (such as when operating in the first mode) then the cooling can be power-efficient, but, the humidity removal capability can be lower than desired. However when operating in the second mode, the lower suction line pressure means that there will also be a low evaporator saturation temperature. The low evaporator saturation temperature, in turn, promotes the condensation of water vapor in the air flow passing through evaporator, thereby dehumidifying the air.

One skilled in the art would understand how the control subunit 120 could be programmed to selectively turn on or off the compressors 152, 154, 156, 158 together or individually in coordination with controlling the flow of refrigerant to evaporator stages (e.g., stages 110, 112, 114 in some embodiments) as part of operating in the first or second modes.

In some embodiments, it is desirable for the system 100 to have additional humidity conditioning capabilities. For instance, in some embodiment, the control subunit 120 is further configured to operate the evaporator subunit 105 in a freeze-protection mode. In this mode, the control subunit 120 causes the refrigerant to circulate through the first evaporator stage 110 and not through other evaporator stages (e.g., the second or third stages 112, 114). In some cases, for instance, the control subunit 120 is configured to switch the evaporator subunit 105 from the second mode to the freeze-protection mode when the refrigerant in one or more suction lines 145, 149 leaving the evaporator subunit 105 is below a target temperature or pressure. As illustrated in FIG. 1 the suction lines 145, 149 can each be coupled to a compressor subunit 150 (e.g., comprising one or more compressors 152, 154, 156, 158). For instance, the evaporator saturation temperature or the pressure can be monitored by coupling a sensor 160 (e.g., temperature or pressure sensor) to the suction lines 145, 149 and if the temperature or pressure below the target value (e.g., a temperature at, or a few degrees above, the freezing point of water, or a pressure of 95 psig, in some cases) then the freeze-protection mode is made operative. Consequently, the refrigerant pressure increase, and therefore the evaporator saturation temperature increases thereby increasing the temperature of the suction lines 145, 149, and, any frozen water condensate formed on the lines is melted.

In some embodiments, to further provide humidity conditioning capabilities, further including a reheater subunit 162 located in the air flow pathway 115 after the evaporator subunit 105. That is, substantially all the air reaching the reheater subunit 162 has passed through the evaporator subunit 105. In some embodiments a major surface 165 of the reheater subunit 162 is preferably substantially perpendicular to the airflow pathway 115. Embodiments of the rejecter subunit 162 can be configured with fins and coils similar to the evaporator stages 110, 112, 114 as described above. One skilled in the art would be familiar with the various configurations that the reheater subunit 162 may have. Non-limiting examples are presented non-limiting example configurations of reheater subunits are presented in U.S. Pat. Nos. 6,427,461 and 6,644,049, and, U.S. patent application 2006/0273183, all of which are incorporated by reference herein in their entirety. The reheater subunit 162 can be coupled to a refrigerant flow via inlet and outlets lines 167, 168, 169.

The reheater subunit 162 is configured to receive the refrigerant from a compressor (e.g., one the compressors 158 of compressor subunit 150) or from a condenser (e.g., a condenser subunit 170 having one or more condenser stages 172, 174, 176, 178) of the system 100. The refrigerant from the compressor or condenser is heated and therefore the air passing through the reheater subunit 162 reheats the dehumidified air that has passed through the evaporator subunit 105.

For instance, in some embodiments, the control subunit 120 is configured to cause the refrigerant to circulate through the reheater subunit 162 when the system 100 is operating in the second mode. Operating the reheater subunit 162 during the operation of the second mode can advantageously supplement humidity control when, e.g., the enclosed space has a very high humidity (e.g., as sensed by a humidity monitor located in the enclosed space).

In some preferred embodiments, the reheater subunit's major surface 165 is fully or partially covered by the major surface 117 of the of the first evaporator stage 110. Such an embodiment is preferred in cases where the reheater subunit 162 is only operated when evaporator subunit 105 is operating in mode 2. For instance, because the second evaporator stage 112 is not being operated in the second mode, there would be no advantage in having the reheater subunits major surface 165 covered by the second evaporator stage 112. In some embodiments, however, the second evaporator stage 112 can cover the reheater subunit 162.

Some embodiments of the system 100 can further include one or more additional evaporator stages. For instance, FIG. 3 presents an alternative embodiment of the evaporator subunit 105 of the system 100, corresponding to the same portion of the system 100 depicted in FIG. 2.

For example, as illustrated in FIG. 3, some embodiments of the evaporator subunit 105 further include a fourth evaporator stage 310. The fourth evaporator stage 310 is adjacent to the third evaporator stage 114. The fourth evaporator stage 310 has a major surface 315 that is substantially parallel to the major surface 119 of the third evaporator stage 310. In some cases, the control module causes the refrigerant to not circulate through the fourth evaporator stage in either the first mode or the second mode. That is, the fourth evaporator stage 310 is only used when the evaporator subunit 105 is operated in the third full cooling mode. As illustrated in FIG. 3, in some cases, e.g., to provide a uniform rectangular outer shape to the evaporator subunit, 105 the major surface 315 of the fourth evaporator stage 310 can cover the major surface 118 of the second evaporator stage 112. However in other embodiments, the size of the fourth stage's major surface 315 can be larger or smaller than the third stage's major surface.

As also illustrated in FIGS. 2 and 3, in some embodiments, the evaporator stages 110, 112, 114, 310 are part of a single unitary structure. That is, a solid continuous structure defines the evaporator stages, although, there can be separate refrigerant flows into and out of the, stages. In some cases providing a single unitary structure can be less costly to manufacture and can occupy less space in the system 100. However in other embodiments such as shown in FIG. 4, the first, second and third evaporator stages 110, 112, 114 can be separated from each other by gaps 410. For instance, the evaporator stages 110, 112, 114 can be separately manufactured and then assembled in the system 100.

As further illustrated in FIG. 1, in some cases, each of the evaporator stages 110, 112, 114 is coupled to a different compressor 152, 154, 156. In still other cases, the evaporator stages 110, 112, 114 are connected to different sources of the refrigerant that are not co-mingled. For instance the evaporator stages 110, 112, 114 can be each connected to different compressor 152, 154, 156 and to different condenser stages 172, 174, 175, 178.

In some cases, a coil 132 of the third evaporator stage 114 has a smaller total effective surface area than a coil 132 of the first evaporator stage 110. For instance, as further illustrated in FIG. 1, in some cases, the third evaporator stage 114 has a single row of coils 132 and the first evaporator stage has more than one row of coils 132. In some cases, e.g., so that the evaporator subunit has a uniform rectangular outer shape (e.g., including a uniform thickness 210; FIG. 1), the second evaporator stage 112 can have a number of rows of coils 132 that is equal to the total number of coils 132 in the first and third evaporator stages 110, 114. Similarly, when there is a fourth evaporator stage 310 (FIG. 3), in some cases, a coil 132 of the fourth evaporator stage 310 has a smaller total effective surface area than a coil 132 of the second evaporator stage 112. For instance, the fourth stage 310 can have the same number of rows of coils 132 as the third stage 114, and, the first and second stages 110, 112 can have the same number of rows of coils 132 as each other.

In some embodiments, the system 100 is configured as a roof-top unit. However, other embodiments of the system 100 can be configured for other types of cool applications where refrigerant is used to cool the air transferred to an enclosed space.

Another embodiment of the present disclosure is a method of conditioning air within an enclosed space. FIG. 5 presents a flow diagram of an example method 500 of conditioning air within an enclosed space, such as implemented by any of the systems 100 depicted in FIGS. 1-4.

With continuing reference to FIGS. 1-4 throughout, the example method 500 presented in FIG. 5 includes, a step 505 of sensing a temperature or a humidity of an enclosed space. The method 500 also comprises a step 510 of determining whether or not a space conditioning system 100 coupled to the enclosed space can reduce the temperature or said humidity to a target value when using the system under partial load conditions.

When it is determined in step 510 that using the system under partial load conditions can achieve the target conditions, the method 500 comprises a step 515 of operating an evaporator subunit 105 of the system 100 in one of two partial load modes. In a first mode (step 520), a refrigerant is circulated through a first evaporator stage 110 and a second evaporator stage 112, but not through a third evaporator stage 114 of the evaporator subunit 105. In a second mode (step 525), the refrigerant is circulated through the first evaporator stage 110 and the third evaporator stage 114, but not through the second evaporator stage 112.

As discussed above in the context of FIGS. 1-2, the first and second evaporator stages 110, 112 are adjacent to each other, and, the first and second evaporator stages 110, 112 have major surfaces 117, 118 that are substantially parallel to each other. The third evaporator stage 114 is located in an airflow pathway 115 through the system 100 before the first evaporator stage 110, and, a major surface 119 of the third evaporator stage 114 covers the major surface 117 of the first evaporator stage 110. Major surfaces 117, 118, 119 of each of the evaporator stages 110, 112, 114 are substantially perpendicular to the airflow pathway 115.

In some embodiments, when it is determined in step 510 that using the system under partial load conditions can not achieve the target conditions, the method 500 comprises a step 530. Operating the evaporator subunit 105 in a third mode (step 530 (e.g., a full cooling mode in some cases) wherein the refrigerant is circulated through all of the evaporator stages 110, 112, 114.

In some embodiments, the evaporator subunit is operated in the first mode, in step 520, when the temperature of the enclosed space is above a target value. In some embodiments the evaporator subunit is operated in the first mode, in step 522 when a high energy efficiency (e.g., a higher efficiency than when operating in the second mode) is desired. In some embodiments, the evaporator subunit 105 is operated in the second mode in step 525 when a high humidity removal capacity is desired (e.g., a higher humidity removal capacity than when operating in the first mode). Such an operating mode could be triggered, e.g., when the humidity (or equivalently, a high latent capacity) of the enclosed space is above a target value.

Some embodiments further include a step 535 of determining whether to switch the evaporator subunit 105 from the second mode (step 525) to a freeze-protection mode in step 540 when a saturation temperature or pressure of the refrigerant, in one or more suction lines 145, 149 leaving the evaporator subunit 105, is less than a target value. Operating in the freeze-protection mode (step 540), includes circulating refrigerant through the first evaporator stage 110 but not through the other evaporator stages 112, 114.

Some embodiments further include a step 545 of circulating the refrigerant through a reheater subunit 162 located in the airflow pathway 115 after the evaporator subunit 105. In some cases, for instance, operating the reheater subunit in step 545 can be in response to sensed humidity of the enclosed space (step 505) being above a target value, e.g., while the temperature of enclosed space is equal to or less than a target value. Once the reheat operation in step 545 starts, the control subunit 120 could switch the operational mode (step 525) to maximize humidity removal.

One skilled in the art would appreciate that the method 500 could include additional steps to condition the air within the enclosed space. For instance, in some embodiments, any of steps 505 to 545 could be controlled a control subunit 120 of the system 100 that receives inputs from various sensors, including enclose space humidity or temperature sensors sensor and communicates operating commands to the evaporator subunit 105, condenser subunit 150, reheater subunit 162, condenser subunit 170, or other components (e.g., air blower) of the system 100.

Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments. 

What is claimed is:
 1. A space conditioning system for conditioning air within an enclosed space, comprising: an evaporator subunit, said evaporator subunit including at least three evaporator stages in a pathway of air flow through said evaporator subunit, wherein: first and second ones of said evaporator stages are adjacent to each other and have major surfaces that are substantially parallel to each other a third one of said evaporator stages is located in said pathway before said first evaporator stage and a major surface of said third evaporator stage covers said major surface of said first evaporator stage in a same direction of said pathway, and said major surfaces of each of said evaporator stages are substantially perpendicular to said airflow pathway; and a control subunit configured to operate said evaporator subunit in at least one of two partial load cooling modes, wherein: in a first mode, said control subunit causes refrigerant to circulate through said first evaporator stage and through said second evaporator stage but not through said third evaporator stage, and in a second mode, said control subunit causes said refrigerant to circulate through said first evaporator stage and through said third evaporator stage but not through said second evaporator stage.
 2. The system of claim 1, wherein a total effective surface area of a coil of said second evaporator stage is greater than a total effective surface area of a coil of said third evaporator stage.
 3. The system of claim 1, wherein, an evaporator saturation temperature of said refrigerant in one or more suction lines leaving said evaporator subunit is higher when said subunit is operating in said first mode as compared to said second mode.
 4. The system of claim 1, wherein an enclosed space that said system is configured to condition has a greater humidity removal capacity when said evaporator subunit is operating in said second mode than when operating in said first mode.
 5. The system of claim 1, wherein said control subunit is further configured to operate said evaporator subunit in a freeze-protection mode, wherein said control subunit causes said refrigerant to circulate through said first evaporator stages and not through other said evaporator stages.
 6. The system of claim 5, wherein said control subunit is further configured to switch said evaporator subunit from said second mode to said freeze-protection mode when said refrigerant in one or more suction lines leaving said evaporator subunit is below a target temperature or pressure.
 7. The system of claim 1, further including a reheater subunit located in said flow pathway after said evaporator subunit, said reheater subunit configured to receive said refrigerant from a compressor or a condenser of said system.
 8. The system of claim 7, wherein said control subunit is further configured to cause said refrigerant to circulate through said reheater subunit when said system is operating in said second mode.
 9. The system of claim 1, wherein said evaporator subunit further includes a fourth evaporator stage, wherein said fourth evaporator stage is adjacent to said third evaporator stage, said fourth evaporator stage has a major surface that is substantially parallel to said major surface of said third evaporator stage, and, said control module cause said refrigerant to not circulate through said fourth evaporator stage during either said first mode or said second mode.
 10. The system of claim 1, wherein said evaporator stages are part of a single unitary structure.
 11. The system of claim 1, wherein said first, second and third evaporator stages are separated from each other by gaps.
 12. The system of claim 1, wherein each of said evaporator stages are connected to a different compressor.
 13. The system of claim 1, wherein a coil of said third evaporator stage has smaller total effective surface area than a coil of said first evaporator stage.
 14. The system of claim 1, wherein said system is configured as a roof-top unit.
 15. A method of conditioning air within an enclosed space, comprising: sensing a temperature or a humidity of an enclosed space; determining whether or not a space conditioning system coupled to said enclosed space can reduce said temperature or said humidity to a target value when using said system under partial load conditions; when using said system under partial load conditions can achieve said target conditions, operating an evaporator subunit of said system in one of two modes, wherein: in a first mode, a refrigerant is circulated through a first evaporator stage and a second evaporator stage, but not through a third evaporator stage of said evaporator subunit, and in a second mode, said refrigerant is circulated through said first evaporator stage and said third evaporator stage but not through said second evaporator stage, and: said first evaporator stage and second evaporator stage are adjacent to each other, and have major surfaces that are substantially parallel to each other; said third evaporator stage is located in an airflow pathway through said system before said first evaporator stage and a major surface of said third evaporator stage covers said major surface of said first evaporator stage, and said major surfaces of each of said evaporator stages are substantially perpendicular to said airflow pathway.
 16. The method of claim 15, wherein said evaporator subunit is operated in said first mode when a high energy efficiency is desired.
 17. The method of claim 15, wherein said evaporator subunit is operated in said first mode when said temperature of said enclosed space is above a target value.
 18. The method of claim 15, wherein said evaporator subunit is operated in said second mode when a higher humidity removal capacity is desired.
 19. The method of claim 18, further including switching said evaporator subunit from said second mode to a freeze-protection mode when a saturation temperature or pressure of said refrigerant in one or more suction lines leaving said evaporator subunit is less than a target value, wherein operating in said freeze-protection mode includes circulating said refrigerant through said first evaporator stage but not through said other evaporator stages.
 20. The method of claim 18, further including circulating said refrigerant through a repeater subunit located in said airflow pathway after said evaporator subunit. 